Include geometry data in DIOCGPART so fdisk can use it instead of trying

Include geometry data in DIOCGPART so fdisk can use it instead of trying
to read a faked disklabel.

Change MAKEDEV to create 'compatibility slice' devices, e.g. da0s0a,
da0s0b, etc.  Previously the compatibility slice devices were e.g. da0a, da0b,
and there was no 'whole slice' device for the compatibility slice at all,
meaning one couldn't disklabel it.  Now there is, e.g. da0s0.